Method of shift control in an automatic transmission

ABSTRACT

A method of shift control in an automatic transmission having a torque converter and a shift-gear mechanism, wherein a shift map has a shift-up line and a shift-down line between each gear ratio change which is used to conduct the shift control, with a hysteresis zone being established between each shift-up line and the shift-down line. The method includes the steps of shifting down a speed range when a point representing a traveling state on the shift map moves in the lower vehicle speed direction and crosses the shift-down line, shifting up a speed range when the point moves in the higher vehicle speed direction and crosses the shift-up line, and shifting down the speed range when the point is in the hysteresis zone after a ratio of the torque converter has been shift and also the speed below a threshold level for more than a predetermined time.

BACKGROUND OF THE INVENTION

The present invention relates to a method of shift control in an automatic transmission to automatically shift the speed range corresponding to changes in engine power and vehicle speed.

In an automatic transmission, the speed range is automatically shifted corresponding to changes in traveling state to attain desirable traveling characteristics. A point representing a traveling state is shown on a shift map point representing a traveling state is shown on a shift map having shift-up lines and shift-down lines which are established based on vehicle speeds and engine powers (or throttle openings, intake vacuum pressures etc.). The shift control is conducted based on the movement of the point representing the traveling state on the shift map. Such a control method as described above is disclosed in the Japanese Patent Laid-Open Publication No. 61(1986)-189354.

The shift map is established considering many road conditions such as open roads, highways and mountainous roads. Because of so many conditions to be considered, the establishment of the map is complicated.

The shift lines on the shift map are established so as to cope with the many conditions. In order to fully utilize the engine power, the speed range is shifted up when a point representing the traveling state moves toward higher speed direction crossing the shift line, and it is shifted down when the point moves toward lower speed direction crossing the shift line. If the up-shift and down-shift are determined based on one common shift line, the up-shift and the down-shift can happen frequently to deteriorate the driving feeling when the point moves in the vicinity of the shift line. Accordingly, the shift-up line is usually established on higher speed side than the shift-down line. A hysteresis zone established between the lines can moderate the frequency of the shifts.

It is desirable to include a hysteresis zone to reduce the frequency of the shifts. However, if the zone is too broad, the following problem may arise. For example, when the speed range is shifted up during running on an ascending slope, the downshift may not be conducted until the vehicle speed is greatly decreased. Accordingly, the driving force becomes poor to deteriorate the driveability. Further, the torque converter is operated in a range of low speed ratio (speed ratio=output speed/input speed) to create large slip and heat in the torque converter.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of shift control wherein excessively frequent shifts can be moderated.

Another object of the present invention is to provide a method of shift control wherein the speed range can be shifted down quickly when a driving force is required to be increased.

In order to achieve the objects, in the method of shift control according to the present invention, up-shifts and down shifts of the speed range are normally controlled based on a shift map having shift-up lines, shift-down lines and hysteresis zones therebetween but when a point representing a traveling state is in the hysteresis zone after an up-shift and also a speed ratio of the torque converter has been below a threshold level for more than a predetermined time, the speed range is shifted down.

In the shift control based on the above method, the speed range is normally shifted up or down when the point crosses the shift-up line or the shift-down line. Since a hysteresis zone is established between the lines, excessively frequent shifts are prevented. However, when the speed ratio of the torque converter is lowered because of an increase of traveling load which can happen during running on an ascending slope, for example, it is judged whether the speed ratio is below the threshold level or not. If it has been below the threshold level for more than the predetermined time and the point representing the traveling state is in the hysteresis zone, the speed range is shifted down even if the point does not cross the shift-down line toward low speed direction. Accordingly, the driving force is increased quickly, increasing the engine speed and the pump (the impeller) speed of the torque converter. As a result, the speed ratio is increased, increasing pump absorption torque and decreasing heat generation.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying wherein:

FIG. 1 is a diagrammatic view of an automatic transmission whose speed range is controlled by a method in accordance with the present invention.

FIG. 2 is a hydraulic circuit diagram of a control valve used in the above automatic transmission.

FIG. 3 is a graph showing a shift map used for the above method.

FIG. 4 is a flow chart showing the above method.

FIG. 5 is a flow chart showing intake vacuum or manifold pressure as an alternate, interchangeable measure of engine power.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagrammatic view illustrating the construction of an automatic transmission whose speed range is shifted based on a method according to the present invention.

In this transmission AT, the engine output power is transmitted from the engine output shaft 1 to the transmission output shaft 6 through a torque converter 2 and a speed reduction mechanism 10 having a multiplicity of gear trains by which the rotational speed is changed. Specifically, the output power of the torque converter 2 is transmitted to the main shaft 3. Then it is transmitted to the counter shaft 4 arranged in parallel with the main shaft 3 after it is changed in speed by means of one of five gear trains disposed between the main shaft 3 and the counter shaft 4. The output power is further transmitted to the output shaft 6 through output gear train 5a and 5b disposed between the counter shaft 4 and the output shaft 6.

The five gear trains disposed between the main shaft 3 and the counter shaft 4 include a first range gear train 11a and 11b, a second range gear train 12a and 12b, a third range gear train 13a and 13b, a fourth range gear train 14a and 14b, and a reverse range gear train 15a, 15b and 15c. The gear trains are respectively provided with hydraulically operable clutches 11c, 12c, 13c 14c, and 15d to allow the transmission of the power through the gear trains. The first range gear 11b is provided with a one-way clutch 11d. Thus, by selectively putting one of these five gear trains in operation, a desirable gear ratio may be achieved.

The five hydraulic clutches 11c through 15d mentioned above are selectively operated by the pressurized hydraulic fluid supplied from a hydraulic control valve 20 through hydraulic lines 21a, 21b, 21c, 21d and 21e.

The operation of the hydraulic control valve 20 is controlled in accordance with the operation of a manual valve-spool 25, which is connected to a shift lever 45 by means of a wire 45a, and the operation of two solenoid valves 22, 23. The shift lever is manually operated by a driver.

The solenoid valves 22, 23 are turned on and off in response to operational signals sent from a controller 30. A vehicle speed signal detected by a vehicle speed sensor 32 based on the rotation of the output gear 5b, a throttle opening signal representing an opening of an engine throttle 41 detected by a throttle opening sensor 33, an input speed signal representing a pump rotational speed of the torque converter 2 detected by a rotational speed sensor 34, and an output speed signal representing a turbine rotational speed of the torque converter 2 detected by a rotational speed sensor 35 based on the rotation of the gear 13a are sent to the controller 30 through signal lines 32a, 33a, 34a, 35a. The engine throttle 41 is connected to the throttle pedal 43 by means of a wire 42. Therefore, the depression of the throttle pedal can be found based on the throttle opening detected. The speed ratio (e=input speed/output speed) of the torque converter 2 is determined by the controller 30 based on the input speed signal and the output speed signal detected.

The hydraulic control valve 20 is described hereinafter referring to FIG. 2.

In the control valve 20, the operational fluid supplied by the pump 8 from an oil sump 7 is led through a line 101 into a regulator valve 50 which functions to regulate the fluid pressure to a predetermined line pressure. The fluid having the line pressure is led to the manual valve 25 through the line 110. Then the fluid is supplied, via various valves in the control valve 20, to a selected one of hydraulic clutches 11c, 12c, 13c, 14c and 15d in accordance with changes of traveling state thereby controlling the operation of the clutch.

These various valves in the control valve 20 will be described below. A relief valve 52 is disposed downstream of the regulator valve 50 in a line 102 and prevents the pressure of the fluid for lubrication supplied to the transmission through line 102 from exceeding a predetermined pressure. A modulator valve 54 reduces the line pressure of the fluid supplied through a line 103 to a predetermined modulator pressure, and supplies this operating fluid having the modulator pressure to a lock-up clutch control circuit (not shown), via a line 104, to control the lockup clutch in the torque converter 2. The fluid having modulator pressure is also supplied to the first and second solenoid valves 22, 23 through a line 105 for shift control.

A manual valve 25 is operable in response to the shift lever manipulated by the driver, and may take either one of 6 positions, P, R, N, D, S, and 2. The fluid having the line pressure from the line 110 is selectively supplied to the lines 25a through 25g in correspondence with the above positions.

A 1-2 shift valve 60, a 2-3 shift valve 62, and a 3-4 shift valve 64 are operated when the manual valve 25 is in one of D, S, or 2 positions by the action of the modulator pressure supplied through lines 106a to 106f in accordance with the ON-OFF actuation of the first and the second solenoid valves 22 and 23. These valves 60, 62, 64 are provided for controlling the supply of the line pressure fluid to the first through the fourth clutches 11c, 12c, 13c and 14c.

The lines 106a, 106b communicate with the first solenoid valve 22 and also with the line 105 through an orifice 22a. When a power supply to the solenoid valve 22 is "off", the drain port in the solenoid valve 22 is closed, thereby supplying the operational fluid having the modulator pressure from the line 105 to the lines 106a, 106b. When the power supply is "on", the drain port is opened, thereby depressurizing the fluid in the lines 106a, 106b to "zero". The lines 106c to 106f communicate with the second solenoid valve 23 and also with the line 105 through an orifice 23a. When a power supply to the solenoid valve 23 is "off", the drain port in the solenoid valve 23 is closed, thereby supplying the operational fluid having the modulator pressure from the line 105 to the lines 106c to 106f. When the power supply is "on", the drain port is opened, thereby depressurizing the fluid in the lines 106c to 106f to "zero".

The line 106a is connected to the right end of the 1-2 shift valve 60. The line 106b is connected to the right end of the 2-3 shift valve 62. The line 106c is connected to the left end of the 1-2 shift valve 60. The line 106d is connected to the manual valve 25. The line 106e is connected from the manual valve 25 to the right end of the 3-4 shift valve 64. The line 106f is connected from the manual valve to the left end of the 2-3 shift valve 62. The lines 106e, 106f can communicate with the second solenoid valve 23 via the manual valve 25 and the line 106d. When the supply of the fluid having the modulator pressure from the line 105 to the lines 106a to 106f is controlled by the on-off operational control of the first and the second solenoid valves 22, 23, the operations of the 1-2, 2-3 and 3-4 shift valves can be controlled. As a result, the fluid having the line pressure supplied from the line 110 via the manual valve 25 can be selectively supplied to the hydraulic clutches 11c, 12c 13c and 14c to realize a desirable shift control.

The control valve 20 includes a first, a second, a third and a fourth orifice control valve 70, 72, 74, 76. These orifice control valves relieve the hydraulic pressure in the pre-shift clutch during shifting in a manner timed to the building up of the hydraulic pressure in the post-shift clutch. The first orifice control valve 70 relieves the pressure in the hydraulic chamber of the third clutch 13c at the time of down-shift from the third speed range to the second. The second orifice control valve 72 relieves the pressure in the hydraulic chamber of the second clutch 12c at the time of up-shift from the second speed range to the third or from the second to the fourth. The third orifice control valve 74 relieves the pressure in the hydraulic chamber of the fourth clutch 14c at the time of down-shift from the fourth speed range to the third or from the fourth to the second. The fourth orifice control valve 76 relieves the pressure in the hydraulic chamber of the third clutch 13c at the time of up-shift from the third speed range to the fourth.

The hydraulic chambers of the above clutches 11c, 12c, 13c and 14c respectively communicate with pressure-receiving chambers of accumulators 81, 82, 83 and 84. Lines 121, 122, 123 and 124 respectively communicate with back-pressure chambers of the accumulators 81, 82 83 and 84. Piston members 81a, 82a, 83a and 84a respectively divide the spaces in the accumulators into the pressure-receiving chambers and the back-pressure chambers which stand opposite to each other. The lines 121, 122, 123 and 124 communicate with a linear solenoid valve 56 via lines 120a, 120b and 120.

The linear solenoid valve 56 includes a linear solenoid 56a. The operational force of the solenoid 56a can be controlled by controlling current-flow through the solenoid 56a to control the hydraulic pressure of the fluid supplied to the line 120. Therefore, the control of the current-flow through the linear solenoid 56a allows control of the hydraulic pressures in the back-pressure chambers of the accumulators 81 through 84. As a result, the hydraulic pressure in the engaging clutch (post-shift clutch) can be controlled during shifting.

In the hydraulic control valve 20 as described above, operation of the manual valve 25 in accordance with operation of the shift lever 45 and on-off operation of the solenoid valves 22 and 23 actuate the above-mentioned valves to control selective supply of the line pressure to the hydraulic clutches 11c, 12c, 13c and 14c, realizing an automatic shift control.

Next, the method of shift control in the above transmission is described hereinafter.

The shift control is carried out in response to the shift ranges which are determined by the manual valve 25 of the hydraulic control valve 20. The shift ranges include P, R, N, D, S and 2 ranges. In P-range or N-range, all the hydraulic clutches 11c through 15d are disengaged creating a neutral state of the transmission. In R-range, the reverse clutch 15d is engaged establishing the reverse range. At D-range, S-range or 2-range, the shift control is carried out based on a shift map.

As an example, the shift control based on the shift map in D-range is described hereinafter. As shown in FIG. 3, the shift map which shows the relation between the vehicle speed V and the throttle opening in θth in D-range has a shift-up line U₁ and a shift-down line D₁ between the first speed range and the second speed range, a shift-up line U₂ and a shift-down line D₂ between the second speed range and the third speed range, and a shift-up line U₃ and a shift-down line D₃ between the third speed range and the fourth speed range. The shift-up lines U₁, U₂ and U₃ are respectively established at a higher vehicle speed side than the shift-down lines D₁, D₂, D₃, creating hysteresis zones H₁, H₂ and H₃.

The present traveling state is determined by the controller 30 based on a vehicle speed signal and a throttle opening signal sent through the lines 32a, 33a and is expressed as a point on the shift map. The movement of the point is traced on the map. When the point crosses the shift-up line toward high speed direction or the shift-down line toward low speed direction, operational signals to shift up or to shift down are sent from the controller to the solenoid valves 22, 23 through the signal lines 31a, 31b. The on-off actuations of the solenoid valves 22, 23 actuate the hydraulic control valve 20. As a result, the pressurized fluid is selectively supplied to the hydraulic clutches 11c, 12c, 13c and 14c to shift up or shift down.

The shifts (shift-up and shift-down) between the third speed range and the fourth speed range in accordance with the movement of the point representing the traveling state on the shift map is explained hereinafter.

When the vehicle speed is increased with a constant throttle opening during running on a flat road, the point A₁ representing the initial traveling state (the speed range is the third range at this time) moves rightward on the shift map toward the point A₂. During this movement, the point crosses the shift-up line U₃ at point A₃ and the speed range is shifted up from the third range to the fourth.

When the vehicle speed is decreased with a constant throttle opening during running on an ascending road, for example, the point B₁ representing initial traveling state moves leftward on the shift map toward the point B₂. During this movement, the point crosses the shift-down line D₃ at point B₃ and the speed range is shifted down from the fourth speed range to the third.

Since the shift-down line D₃ is placed on a lower speed side than the shift-up line U₃ establishing the hysteresis zone H₃, the vehicle speed at the down-shift is lower than that at the upshift if the throttle opening is not changed. Accordingly, even if the vehicle speed is reduced slightly after the shift-up, the shift-down is not introduced, thereby preventing frequent shifts.

The above-mentioned control is an ordinary control and an interruption control can be conducted in the method according to the present invention. The method of the interruption control comprises a step of shifting down the speed range when the point representing the traveling state is in the hysteresis zone and also the speed ratio of the torque converter has been below a threshold level for more than a predetermined time even if the point does not cross the shift-down line. This interruption control is described hereinafter in connection with FIGS. 3 and 4.

Let's consider a case where the initial point B₁ (vehicle speed: V₁, throttle opening: θ₁) at the fourth speed range moves to the point B₄ (vehicle speed: V₂, throttle opening: θ₁).

First, it is determined whether the speed ratio "e" of the torque converter is less than a threshold value "e_(SD) " or not (step S1 in FIG. 4). If "e" is equal to or greater than "e_(SD) " (e>=e_(SD)), a timer T_(SD) for measuring a predetermined time "t_(SD) " is set at Step S11 and the shift flag F_(sa) is kept at "zero" in Step S14. The timer T_(SD) has a counter whose number is decreased with time after it is set at Step S11 and the number of the counter becomes "zero" when the predetermined time "t_(SD) " has elapsed (T_(SD) =0).

If "e" is less than "e_(SD) " (e<e_(SD)), it is judged whether the timer T_(SD) has become zero (T_(SD) =0) or not. In other words, it is judged whether or not the speed ratio has been below the threshold level "e_(SD) " for more than the predetermined time t_(SD). If the timer T_(SD) has not become zero yet, the shift flag F_(sa) is kept at "zero" Step S14. When the timer T_(SD) becomes zero (T_(SD) =0), a value So which represents the present speed range (the fourth speed range) is stored in a buffer memory B_(SH) at Step S3.

Then the hysteresis judging speed V_(DB) at the point P1, which is on the shift-up line U(B_(SH) -1) from the pre-shift speed range (the third speed range) to the post-shift speed range (the fourth speed range) and corresponds to the present throttle opening θ₁ in FIG. 3, is determined. The hysteresis judging speed V_(DB) is a function of the value (B_(SH) -1) and the throttle opening θ₁ {U(B_(SH) -1),θ₁ }. The present vehicle speed V is compared with the hysteresis judging speed V_(DB) at Step S5.

In the case that the present speed V is equal to or greater than the judging speed V_(DB) (V>=V_(DB)), since the point representing the traveling state is outside the hysteresis zone H₃ as shown in FIG. 3, the control goes to Step S7. On the other hand, in the case that the present speed V is less than the judging speed V_(DB) (V<V_(DB)), since the point is in the hysteresis zone H₃, the control goes to Step S6 where a value representing a lower speed range (the third speed range) is stored in the buffer memory B_(SH). If the present speed is lower than the speed corresponding to the shift-down line D₃, the point is also outside the hysteresis zone H₃. In such a case, however, the speed range is shifted down based on the ordinary control.

When the value representing the lower speed range is stored in the buffer memory B_(SH) at Step S6, a shift-down control could be conducted based on the value in the buffer memory. However, since the shift-up line U₂ from the second speed range to the third may be established in the hysteresis zone H₃, the control goes from Step S6 back to Step S4 where it is judged whether or not the point representing the present traveling state is in the hysteresis zone H₂ established between the second speed range and the third. If the point is in the hysteresis zone H₂, the value in the buffer memory B_(SH) is replaced with a value representing a further lower speed range (the second speed range).

In the above control flow from Step S4 to S6, a value representing the fourth speed range is stored in the buffer memory B_(SH) when the point representing the traveling state is on the higher speed side of the shift-up line U₃, a value representing the third speed range is stored in the memory B_(SH) when the point is in the hysteresis zone H₃ and on the higher speed side of the shift-up line U₂, and a value representing the second speed range is stored in the memory B_(SH) when the point is in both the hysteresis zones H₃ and H₂. Then the control goes to Step S7.

In Step S7, it is judged whether or not the value stored in the buffer memory B_(SH) is equal to the value of the present speed range So. If it is equal to the value of the present speed range, the control goes to Step S12 and S14 where the timer T_(SD) is reset and the shift flag F_(sa) is set to zero.

If the value in the buffer memory B_(SH) is not equal to that of the present speed range S_(O), the value in the buffer memory B_(SH) is compared with the value in the filter memory B_(SHO). By this comparison, the value in the buffer memory B_(SH) cannot be used unless the same value is stored in the buffer memory more than twice successively in this control flow. Accordingly, even if a wrong value is stored in the buffer memory because of a noise, a wrong shift based on the wrong value can be avoided. If the value in the buffer memory B_(SH) is not equal to that in the filter memory B_(SHO), the value in the buffer memory is stored in the filter memory at Step S13. Then the shift flag F_(sa) is set to zero.

In the next step of the control flow, if the value in the buffer memory B_(SH) is not changed, this value is equal to the value in the filter memory B_(SHO) (B_(SH) =B_(SHO)). In such a case, the control goes from Step S8 to step S9 where the value in the buffer memory is stored as a reference shift value B_(sa). Then, the shift flag F_(sa) is set to "1" (Step S10).

In cases where the point representing the traveling state moves from the point B₁ to B₄ in FIG. 3, if the speed ratio "e" of the torque converter has been below the threshold level for more than the predetermined time t_(SD) while the point is in the hysteresis zone H₃, the shift flag F_(sa) is set to 1 and the speed range is shifted down immediately based on the reference shift value B_(sa). Accordingly, the speed range is shifted down from the fourth speed range to the third or the second in correspondence with the value in the buffer memory B_(SH).

As previously noted, throttle opening and intake manifold pressure are alternate ways of measuring engine power. FIG. 5 is an alternative embodiment of the flow chart depicted in FIG. 4, using intake manifold pressure in lieu of throttle opening to calculate hysteresis judging speed V_(DB).

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A method of shift control in an automatic transmission having a torque converter and a shift-gear mechanism, wherein a shift map having at least one shift-up line and at least one shift-down line which are established corresponding to engine powers for driving said torque converter and to vehicle speeds is used to conduct said shift control, said shift-up line being established on a higher vehicle speed side than said shift-down line and a hysteresis zone being established between said shift-up line and said shift-down line, comprising the steps of:shifting down to a lower speed range of said shift-gear mechanism when a point representing a traveling state on said shift map moves in a lower vehicle speed direction and crosses said shift-down line; shifting up to a higher speed range when said point moves in a higher vehicle speed direction and crosses said shift-up line; and shifting down to a lower speed range when said point is in said hysteresis zone after a shift of said speed range and also a speed ratio of said torque converter has been below a threshold level for more than a predetermined time.
 2. A method of shift control in an automatic transmission as defined in claim 1; wherein said shift map is expressed based on a relation between vehicle speeds and engine throttle openings.
 3. A method of shift control in an automatic transmission as defined in claim 1; wherein said shift map is expressed based on a relation between vehicle speeds and engine intake vacuum pressures.
 4. A method of shift control in an automatic transmission as defined in claim 1; wherein first, second, third and fourth speed ranges can be set in said shift-gear mechanism and the shift-up lines and the shift-down lines are established between the first speed range and the second speed range, the second speed range and the third speed range, and the third speed range and the fourth speed range respectively.
 5. A method of shift control in an automatic transmission as defined in claim 1 wherein said shifting down to a lower speed range when said point is in said hysteresis zone occurs after a shift-up of said speed range and also a speed ratio of said torque converter has been below a threshold level for more than a predetermined time.
 6. A method of shift control in an automatic transmission as defined in claim 1 wherein said shifting down to a lower speed range when said point is in said hysteresis zone occurs after a shift-down of said speed range and also a speed ratio of said torque converter has been below a threshold level for more than a predetermined time.
 7. A method of shift control in an automatic transmission as defined in claim 1, further comprising the steps of:determing a hysteresis judging speed based on a present throttle opening and said lower speed range and said higher speed range; comparing said hysteresis judging speed with a present vehicle speed as defined by said point representing said traveling state. 